linux-kernel - [PATCH] lib/lz4: explicitly support in-place decompression

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Date:   Sun, 22 Nov 2020 03:10:24 +0800
From:   Gao Xiang <hsiangkao@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Gao Xiang <hsiangkao@...hat.com>,
        Yann Collet <yann.collet.73@...il.com>,
        Miao Xie <miaoxie@...wei.com>, Chao Yu <yuchao0@...wei.com>
Subject: [PATCH] lib/lz4: explicitly support in-place decompression

LZ4 final literal copy could be overlapped when doing
in-place decompression, so it's unsafe to just use memcpy()
on an optimized memcpy approach but memmove() instead.

Upstream LZ4 has updated this years ago [1] (and the impact
is non-sensible [2]), this commit just synchronizes LZ4
upstream code to the kernel side as well.

It can be observed as EROFS in-place decompression failure
on specific files when X86_FEATURE_ERMS is unsupported,
memcpy() optimization of commit 59daa706fbec ("x86, mem:
Optimize memcpy by avoiding memory false dependece") will
be enabled then.

Currently most modern x86-CPUs support ERMS, these CPUs
just use "rep movsb" approach so no problem at all. However,
it can still be verified with forcely disabling ERMS feature...

arch/x86/lib/memcpy_64.S:
        ALTERNATIVE_2 "jmp memcpy_orig", "", X86_FEATURE_REP_GOOD, \
-                     "jmp memcpy_erms", X86_FEATURE_ERMS
+                     "jmp memcpy_orig", X86_FEATURE_ERMS

We didn't observed any strange on arm64/arm platform before,
but it really needs an update for sure considering this
unsafe overlapping memcpy() behavior for LZ4 in-place
decompression.

[1] https://github.com/lz4/lz4/commit/33cb8518ac385835cc17be9a770b27b40cd0e15b
[2] https://github.com/lz4/lz4/pull/717#issuecomment-497818921
Cc: Yann Collet <yann.collet.73@...il.com>
Cc: Miao Xie <miaoxie@...wei.com>
Cc: Chao Yu <yuchao0@...wei.com>
Signed-off-by: Gao Xiang <hsiangkao@...hat.com>
---
Hi Andrew,

Could you kindly consider picking this patch up, although
the impact is EROFS but it touchs in-kernel lz4 library
anyway...

Thanks,
Gao Xiang

 lib/lz4/lz4_decompress.c | 2 +-
 lib/lz4/lz4defs.h        | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
index 00cb0d0b73e1..fa88e13ef00b 100644
--- a/lib/lz4/lz4_decompress.c
+++ b/lib/lz4/lz4_decompress.c
@@ -263,7 +263,7 @@ static FORCE_INLINE int LZ4_decompress_generic(
 				}
 			}

-			LZ4_memcpy(op, ip, length);
+			LZ4_memmove(op, ip, length);
 			ip += length;
 			op += length;

diff --git a/lib/lz4/lz4defs.h b/lib/lz4/lz4defs.h
index c91dd96ef629..673bd206aa98 100644
--- a/lib/lz4/lz4defs.h
+++ b/lib/lz4/lz4defs.h
@@ -146,6 +146,7 @@ static FORCE_INLINE void LZ4_writeLE16(void *memPtr, U16 value)
  * environments. This is needed when decompressing the Linux Kernel, for example.
  */
 #define LZ4_memcpy(dst, src, size) __builtin_memcpy(dst, src, size)
+#define LZ4_memmove(dst, src, size) __builtin_memmove(dst, src, size)

 static FORCE_INLINE void LZ4_copy8(void *dst, const void *src)
 {
-- 
2.18.4